A turbine in a gas turbine engine is formed from a plurality of blade stages coupled to discs that are capable of rotating about an axis. Each stage is formed from a plurality of aerofoil blades spaced circumferentially around a respective disc. Each stage includes a set of non-rotating stators upstream of the rotors.
The turbine blades have tips that are located in close proximity to a seal in the casing that encloses the turbine. A large gap between the tip and the casing decreases the efficiency of the turbine through over-tip leakage. A narrow gap increases the risk of “tip rub” where the tip comes into contact with the seal and causes excessive wear on the components.
The tips of the blades can be coated with abrasive particles such as Cubic Boron Nitride (CBN). The particles help the blade to cut into the seal during the first use of the blade and establish an optimum tip gap. It is desirable for the particles to remain attached to the turbine tip throughout the life of the tip so that the particles can later cut the seal to compensate for blade changes caused, e.g., by creep during the life of the blade.
The particles may be secured to the tip of the aerofoil either by electroplating or by brazing. In both of these methods, the electroplate and braze material are weaker than the alloy of the blade and have a limited ability to mechanically bind the particles.
Alloys for turbines typically have minor amounts of elements such as Hafnium, which improve the high temperature oxidation resistance of the blade. Electroplating is not capable of depositing complex alloys in the tight compositional tolerances required to give desired properties.
Turbines are located downstream of a combustor and are high temperature components. The melting point depressant added to braze alloys render the alloy further unsuitable for application at a blade tip since the blade tips are subject to the highest temperatures.
High temperature creep resistant alloys used to manufacture turbine blades have characteristics that make them prone to cracking on welding. Techniques such as direct laser deposition, where a melt pool is formed in a component to which material is added to form a desired structure, have been developed. The process of welding can cause cracking at brittle grain boundary phases. Where abrasive particles are used their random geometrical alignment within the securing metal can further reduce the bond strength of the securing metal.
Such a technique is further unsuitable to secure abrasive particles as the CBN particles float on the molten pool of much more dense metal resulting in reduced adhesion. It has also been found that CBN particles can be partially decomposed under the intensity of the laser beam used to form the melt pool.